This invention relates to inverted solar cell structures in which the active inversion layer is at the rear of the solar cell away from the incident light direction.
A photovoltaic cell is a device that directly converts photon radiant energy into electrical energy. In a photovoltaic device, photons of sufficient energy react with a semiconductor material to produce positive and negative charge carriers, electrons and holes which can move freely throughout the semiconductor. The object of the device is to collect electrons at one electrical contact and holes at the other electric contact, before they recombine, either at the contacts themselves, elsewhere on the surface, or in the bulk of the semiconductor. To cause these free charge carriers to flow selectively to their respective contacts, the contacts must form a barrier to one type of carrier and permit the opposite carrier to pass freely. Until recently, the most effective selective barrier have been p-n junctions which are formed at the interface between p-type and n-type impurity contained regions of the semiconductor.
If the bulk silicon is of very high quality most of the recombination will occur at the surfaces, especially at the contacts. Conventional doped semiconductor contacts which are used to produce p-n junctions are relatively inferior in this respect. This type of recombination is usually measured in terms of the forward leakage current symbolized as J.sub.o. Recently, it has been shown that bipolar transistors with improved current gain can be produced by making hetero junction emitters of n-type doped SIPOS (Semi-Insulating Polysilicon, a mixture of microcrystalline silicon and silicon dioxide), see e.g. T. Matsushita, N. Ohuchi, H. Hayoshi, and H. Yamoto, Appl. Phys. Lett., Vol. 35, No. 7, (1979). This implies that they had improved the forward leakage current of the emitter. Values of J.sub.o as low as 10.sup.-14 Amps/cm.sup.2 have recently been achieved with this type of heterocontact, which is about two orders of magnitude superior to a conventional p-n junction.